Butt joint for flexible abrasive sheet material



Sept. 24, 1968 w. 1 JOHNSON 3,402,514

BUTT JOINT FOR FLEXIBLE ABRASIVE SHEET MATERIAL Filed NOV. 30, 1966United States Patent Olce 3,402,514 Patented Sept. 24, 1968 3,402,514BUTT JOINT FOR FLEXIBLE ABRASIVE SHEET MATERIAL William L. Johnson,Hanover, Mass., assignor to Abrasive Products, Inc., South Braintree,Mass., a corporation of Massachusetts Filed Nov. 30, 1966, Ser. No.597,904 3 Claims. (Cl. 51-399) This invention relates to joining theends of flexible abrasive coated sheet material as in the fabrication ofbelts, bands, cones and other endless abrasive sheet products. Productsof this invention have butt joints or splices as distinguished from lapjoints, although the joints may, and preferably do run, at an anglebetween 30 and 80 with respect to the edges of the endless material.

While the art has heretofore recognized the advantages, when buttjoining abrasive sheet material, of not increasing the thickness of thecomposite sheet at the joint, lest bumping and jumping of the compositewhile it runs between the work-piece and a platen, drum or back-up rolladversely affect the abrasive finishing, it has sought to do this byrelying either entirely upon the compressibility of the backing materialor upon the actual removal of a portion of the backing material in orderto accommodate a joint material applied to the composite. (See U.S.Patents Nos. 2,391,731, 3,154,897 and 2,733,181.)

The procedure of destroying and removing a part of fibrous backingstructures was admittedly developed because of the unreliability ofrelying upon compression alone to attain the desired unincreasedthickness at the joint, particularly where the backing material wasrelatively incompressible or not sufiiciently compressible to accept theentire thickness of the applied joining material.

The removal method, while attaining the desired unincreased thickness,does have the disadvantage inherent in excising any portion of thebacking liber, namely of weakening the backing structure to the extentof any such removal and increasing the likelihood of creasing at thejoint.

It is thus an object of this invention to provide a butt joint ofreliable maximum thickness not exceeding the thickness of the compositeabrasive material ion each side of the joint but without weakening thecomposite by any removal, abrasion or disruption of any portion `of thefibrous backing structure. Structures of the present invention thusconstitute a radical departure from previous joints.

The present invention is founded upon an appreciation of the fact that,whereas it has heretofore been considered essential to retain the coatedabrasive face of the sheet as intact as possible at the joint, in factinterruption of cutting during the passage of the joint across the workis not disadvantageous so far as good finishing is concerned so long asthe grit side of the joint remains smooth, unbumped and in or below theplane of the cutting surface of the grit on each side of the joint.

It has further been found` that a depression of sufficient depth toaccommodate a strong flush reinforced crossbinding insert of more thansufficient tenacity to maintain the joint intact for the life of theproduct, can be made without reaching the plane of the fibrous backingmaterial, thus leaving the backing structure intact.

Because of the smoothness, flush nature, durability and strength of theinsert after the product is finished, no defects in sanded products arediscernible because of the presence -of the joint which has improvedstrength due to the undisrupted integrity of the backing and maximumthicknesses no greater than those provided when the backing structure ispartially disrupted or removed.

A joint of this invention is shown in cross-section in the accompanyingdrawing, the elements of which will b e referred to in connection withthe following descriptions of examples of products of this invention.

EXAMPLE l A length of conventional woven cloth backed abrasive sheetmaterial having a total thickness of .032. inch was trimmed at its endsso that the straight ends, when brought into abutting relation, made abelt joint extending across the material at a 45 angle. The thickness ofthe filled cloth backing (cotton drill having a thread count of 76 perIinch in the warp and 48 per inch in the fill) was approximately .012inch leaving a combined making coat and sizing (phenol aldehyde) andgrit thickness of approximately .020 inch. Each end of the strip wasthen presented to a diamond grinding wheel to grind off the abrasivegrit to `a depth of about .006 to .007 inch for a distance of about 3/8inch extending back from each end edge.

An insert was prepared as follows:

A sheet of woven fiberglass reinforcement made from monofilament glassroving and having a thickness of about 10 mils and a transientcompressibility without loss of tensile strength of about 500% wasplaced between two sheets of nylon-phenol-aldehyde film, each ofapproximately 3 mill thickness (sold by Chrysler Corporation as C46) andcompressed in a flat laminating press at a temperature not exceeding 350F. and running from 25 seconds at 340 F. up to 75 seconds at 325 F. witha pressure of 500 pounds. This operation compressed the glass and filmcomposite to a thickness of about 7 mils, encapsulating the fibersbetween the overlying polymeric layers.

The ends to be joined 0f the abrasive sheet were placed in abuttingrelation in a jig with the ends about 1&4 inch apart. A piece of theglass reinforcement compressed and encapsulated between the compressedpolymer films was cut on the bias at a 45 angle to give a width of about4%.; inch so that it would just lit into the depression made on eachside of the joint by the previous grinding operation. On the back side,the two marginal edge portions of the cloth backing were overlapped witha narrow strip of the same film in 3 mil thickness. The entire joint wasthen pressed between Teflon coated anvils with the top anvil being at385 F. and the bottom anvil at 350 F. for about ten seconds, being aperiod long enough to flow the polymeric -film between the butt ends andcompress the composite polymeric film and glass reinforcement into apliable matrix, H-shaped in crosssection, adherent to the fibrousbacking ends and having a thickness no greater than the thickness of thestock. This left the upper element of the H-shaped joint flush with thecutting plane of the abrasive grit on each side of the joint, seated inthe ground-away depression, and adherent to the bottom of thedepression; and left the lower parallel element of the H-shapedstructure compressed and merged into the plane of the composite beltstock.

The temperature and the time the film was left between the anvils wasnot enough to completely polymerize but was suflicient to completelythermo-stabilize the synthetic polymeric bonding material so that evenif subsequent temperatures exceeding 375 F. were encountered inoperation, the result would be a further hardening rather than anysoftening of the matrix material which remained heat resistant up to alocalized operating temperature as high as 450 F.

In a reinforced H-shaped joint of this character, the joint will notbreak open on either side nor crease nor hinge even when severely foldedor twisted. Its pliability is excellent because of the fact that thematrix of bonding material is not completely polymerized and does notbecome brittle as the result of application of a high operatingtemperature.

3 EXAMPLE 2 The same procedure was followed as in Example 1 except thatthe backing material was constituted of 130 pound per ream cylinderpaper having a thickness of 10 mils, having an animal hide glue makingcoat, an abrasive layer and an overlying phenolaaldehyde sizing coat,totalling 2() mils in thickness.

Structures .resulting from fabrication in accordance with Examples 1 and2 hereof are illustrated in the accompanying drawing wherein 10designates the fibrous backing material, 12 indicates an adhesive coatbonding a layer 14 of abrasive grit to the backing 1i). 16 indicates thepre-formed insert which is placed in the depression formed by partial orwhole removal of the abrasive grit and adhesive on the grit side of thecomposite. 18 indicates the film which by heat and pressure has beenpartially compressed, partially impregnated into the back side of thefibrous sheet 10, and 20 illustrates the joining leg to complete thematrix of H-shape cross-section. 22 shows the compressed woven glassroving reinforcement encapsulated in the insert 16.

While as described the element 1S is unreinforced, it can, of desired bereinforced, provided the compressibility of the backing will accommodateits combined matrixreinforcement thickness. ln such case very fineplastic or metal or glass filaments may be used, since the main jointstrength is provided by the upper element 16.

The above nylon modified resin has been found to be particularlyefficacious by reason of its temperature stability and anti-frictionalqualities, but it is contemplated that equivalent qualities may bepresent in other nylon or otherwise modified co-polymeric resins, theessential nature of this invention not residing in any particularchemical nature of the synthetic polymeric resin so long as it achievesthe physical requirements for producing a matrix of the type procuredwith the use of the above nylon modified phenolic resin.

What is claimed is:

1. A butt joint for fiexible abrasive grit-coated sheet materialcomprising:

two abutting straight end edges of a composite structure, including afibrous backing bearing a layer of abrasive grit adhesively adherent toat least the upper face thereof,

portions of said composite, marginally adjoining said straight edges onsaid abrasive grit upper face, being recessed to a depth short of saidfibrous backing, and

a pliable substantially thermostable matrix, H-shaped in cross-section,of synthetic polymeric bonding material embracing said composite endedges with one of its pair of parallel elements seated in said upperface recess, and the other of its parallel elements adhered on its innerside to the other under face of said backing,

said matrix having its upper surface substantially flush with thecutting plane of said abrasive grit layer on each side of said joint,

and a layer of woven glass roving compressed and encapsulated within,and being substantially co-extensive with, the upper parallel element ofsaid matrix.

2. A butt joint as claimed in claim 1, wherein the other of saidparallel elements is thinner than the element seated in said upperlrecess and is compressibly merged into the plane of said composite,whereby said matrix is no thicker than the maximum thickness of the saidcomposite structure on each side of said joint.

3. A butt joint as claimed in claim 1, wherein said matrix comprises anylon-phenol-aldehyde polymer.

References Cited UNITED STATES PATENTS 2,391,731 12/1945 Miller et al51-399 2,733,181 1/1956 Riedesel 51-293 2,794,726 4/1957 Riedesel et al.51-297 3,154,897 1l/l964 Howard 51-399 OTHELL M. SIMPSON, PrimmyExaminer.

1. A BUTT JOINT FOR FLEXIBLE ABRASIVE GRIT-COATED SHEET MATERIALCOMPRISING: TWO ABUTTING STRAIGHT END EDGES OF COMPOSITE STRUCTURE,INCLUDING A FIBROUS BACKING BEARING A LAYER OF ABRASIVE GRIT ADHESIVELYADHERENT TO AT LEAST THE UPPER FACE THEREOF. PORTIONS OF SAID COMPOSITE,MARGINALLY ADJOINING SAID STRAIGHT EDGES ON SAID ABRASIVE GRIT UPPERFACE, BEING RECESSED TO A DEPTH SHORT OF SAID FIBROUS BACKING, AND APLIABLE SUBSTANTIALLY THERMOSTABLE MATRIX, H-SHAPED IN CROSS-SECTION, OFSYNTHETIC POLYMERIC BONDING MATERIAL EMBRACING SAID COMPOSITE END EDGESWITH ONE OF ITS PAIR OF PARALLEL ELEMENTS SEATED IN SAID UPPER FACERECESS, AND THE OTHER OF ITS PARALLEL ELEMENTS ADHERED ON ITS INNER SIDETO THE OTHER UNDER FACE OF SAID BACKING, SAID MATRIX HAVING ITS UPPERSURFACE SUBSTANTIALLY FLUSH WITH THE CUTTING PLANE OF SAID ABRASIVE GRITLAYER ON EACH SIDE OF SAID JOINT, AND A LAYER OF WOVEN GLASS ROVINGCOMPRESSED AND ENCAPSULATED WITHIN, AND BEING SUBTANTIALLY CO-EXTENSIVEWITH, THE UPPER PARALLEL ELEMENT OF SAID MATRIX.